1. Field of the Invention
The field of this invention relates generally to frequency dependent quadrature mismatch determination/calibration and compensation therefor, and in particular to quadrature (I/Q) compensation of frequency-dependent response mismatch of transmit and/or receive analog filters.
2. Description of the Prior Art
A primary focus and application of the present invention is the field of wireless telecommunications. In digital wireless telecommunications, an analog carrier signal is digitally modulated by a discrete signal at a transmitter, with a corresponding demodulation and detection performed at the receiver following analog-to-digital conversion of the received analog carrier signal. The digital signals comprise an in-phase signal (or ‘I’, with one example being a cosine waveform) and a quadrature phase signal (or ‘Q’, with an example being a sine wave), which are amplitude and/or phase modulated with a finite number of amplitudes/phases, and then summed.
A typical transmitter performs the following functions: group the incoming data bits into codewords, one for each symbol that will be transmitted; map (modulate) the codewords to, for example, amplitudes of ‘I’ and ‘Q’ signals; oversampling and filtering the digital signal and thereafter digital to analog conversion (DAC) of the ‘I’ and ‘Q’ signals and then with reconstruction filter to suppress the unwanted image. A high frequency carrier waveform then frequency translates the modulated signal to a radio frequency (RF) signal for amplification, further filtering and radiation from an antenna.
A typical receiver performs the following functions: bandpass filtering and automatic gain control of a received wireless signal, frequency down-conversion of the RF signal to equivalent intermediate frequency/baseband/digital ‘I’ and ‘Q’ signals, by mixing the RF signal with a local oscillator signal, sampling and analog-to-digital conversion (ADC) of the down-converted signal and various signal processing of the down-converted ‘I’ and ‘Q’ signals as part of the detection and demodulation process to recover the original transmitted codewords.
Referring to FIG. 1, a known transceiver architecture is illustrated, comprising a receiver 100 and a transmitter 150, shown as distinct circuits for ease of review. The receiver 100 comprises an antenna 102, a low noise amplifier (LNA) 104, two frequency down-conversion quadrature mixers 106, 108, two respective low pass filters (LPFs) 110, 112, two analog-to-digital converters (ADCs) 114, 116 (one for each quadrature path), a combiner 117 arranged to sum the two digital quadrature signals and a baseband (BB) demodulator 118.
Antenna 102 receives a radio frequency (RF) signal, which is fed into the LNA 104. The LNA 104 amplifies the received signal and outputs an amplified signal to each of the frequency down-conversion mixers 106, 108. Frequency down-conversion mixer 106 mixes the amplified signal with a local oscillator signal 107 (I) and outputs the frequency down-converted quadrature signal to low pass filter 110. Similarly, frequency down-conversion mixer 108 mixes the amplified signal with a local oscillator signal 109 (Q) and outputs the frequency down-converted quadrature signal to low pass filter 112. The two local oscillator signals (I) 107 and (Q) 109 are quadrature related (separated in phase by 90°). The low pass filtered signals are input to ADCs 114, 116 respectively, which sample the analog low pass filtered signals at a defined sampling frequency. A digital output from the ADCs 114, 116 is fed via combiner 117 into the BB demodulator 118, e.g. comprising a fast fourier transform (FFT) engine for an orthogonal frequency division multiplex (OFDM) system, which processes the received signal in the digital domain.
In a transmit sense, the transmitter 150 comprises a digital signal processor (DSP) 152, digital-to-analog converters (DACs) 154, 156, LPFs 158, 160, frequency up-conversion mixers 162, 164, power amplifier (PA) 166 and an antenna 168, which may be the same antenna 102 as in the receiver.
In the transmit sense, DSP 152 generates and outputs pairs of digital quadrature signals, which are input to DACs 154, 156. The DACs 154, 156 convert the digital quadrature signals to analog quadrature signals based on their operating sample rate. The resultant analog quadrature signals are input to LPFs 158, 160, which filter out alias components of the analog quadrature signals. Frequency up-conversion mixer 162 mixes the filtered analog quadrature signal with local oscillator signal 163, and frequency up-conversion mixer 164 mixes the filtered analog quadrature signal with local oscillator signal 165. The two resultant mixed signals are combined before being input to power amplifier 166, which amplifies the combined signal prior to transmission.
A known problem with the abovementioned transceiver is the effect of I/Q mismatch, between the respective quadrature paths, which can cause undesirable I/Q imbalance between the two quadrature signal paths. Quadrature mismatch/imbalances are generally caused by one or more of: a gain error between the frequency down-conversion quadrature mixers 106, 108, 162, 164; phase error between the local oscillator signals 107, 109, 163, 165; and any gain error between the ADCs 114, 116 and DACs 154, 156. It is also known that frequency independent parts of the receiver LPFs 110, 112, and the transmitter LPFs 158, 160 also contribute to the gain error. All of these are generally referred to as frequency independent I/Q (FIIQ) mismatch, with the primary contributor typically being due to gain and/or phase mismatch between the quadrature mixers of ‘I’ and ‘Q’ channel.
Further, quadrature imbalances may be caused by frequency-response related components, such as gain and/or phase mismatch between LPFs 110, 112, and 158, 160, referred to as frequency dependent I/Q (FDIQ) mismatch. Thus, referring to the receiver 100 and transmitter 150, if the components of, say, receive LPFs 110, 112 and transmit LPFs 158, 160 are not perfectly matched, then a non-zero (e.g. gain and/or phase mismatch between the) transfer functions would contribute to a leaked output component.
Therefore, it may be advantageous to be able to negate or reduce or minimise any transmitter or receiver induced FDIQ mismatch.